Question

Explain each statement.

a. The${\mathbf{pK}}_{\mathbf{a}}$ of p-nitrophenol is lower than the ${\mathbf{pK}}_{\mathbf{a}}$of phenol (7.2 vs. 10).

b. The ${\mathbf{pK}}_{\mathbf{a}}$of p-nitrophenol is lower than the ${\mathbf{pK}}_{\mathbf{a}}$of m-nitrophenol (7.2 vs. 8.3).

Short answer

a. Due to the delocalization of the negative charge on the nitro group, the conjugate base is further stabilized as shown in the following resonating structures:

Resonating structures for p-nitrophenol

Delocalization of negative charge on the nitro group

Resonating structures for phenol

b. The p-nitrophenol has two resonating structures in which the negative charge is present on the two oxygen atoms, which makes it an even more stable conjugate base.

On the other hand, the m-nitrophenol has the negative charge delocalized only on the benzene ring, as shown in the following resonating structures:

Resonating structures of m-nitrobenzene

Step-by-step solution

The meaning of  pKa

The value of ${\mathbf{pK}}_{\mathbf{a}}$is used to determine the strength of an acid. It is used to determine the strength by which a Bronsted acid holds a proton.

A lower value of ${\mathrm{pK}}_{\mathrm{a}}$ corresponds to a stronger acid, whereas a higher value of ${\mathrm{pK}}_{\mathrm{a}}$corresponds to a weaker acid.

Explanation for the given statements

a. The of the p-nitrophenol is lower than the ${\mathrm{pK}}_{\mathrm{a}}$of phenol. This can be explained in terms of resonance.

After the loss of a proton from p-nitrophenol, the negative charge on the conjugate base of p-nitrophenol is delocalized on the nitro group.

Due to the delocalization of the negative charge on the nitro group, the conjugate base is further stabilized as shown in the following resonating structures:

Resonating structures for p-nitrophenol

Delocalization of negative charge on the nitro group

The p-nitrophenol has two extra structures in which the negative charge is also delocalized on the nitro group.

On the other hand, in the case of phenol, the negative charge is delocalized only round the benzene ring, as shown in the following structure:

Resonating structures for phenol

b. The p-nitrophenol is more acidic than the m-nitrophenol. This is because in p-nitrophenol, the negative charge is delocalized over both benzene and the nitro group, as shown in the following resonating structures:

Resonating structures for p-nitrophenol

The p-nitrophenol has two resonating structures in which the negative charge is present on the two oxygen atoms, which makes it an even more stable conjugate base.

On the other hand, the m-nitrophenol has the negative charge delocalized only on the benzene ring, as shown in the following resonating structures:

Resonating structures of m-nitrobenzene